A linear or two-dimensional ion-processing device such as an ion trap is formed by a set of electrodes coaxially arranged about a central (z) axis of the device and elongated in the direction of the central axis. Typically, each electrode is positioned in the (x-y) plane orthogonal to the central axis at a radial distance from the central axis. The inside surfaces of the electrodes are typically hyperbolic with apices facing inwardly toward the central axis. The resulting arrangement of electrodes defines an axially elongated interior space of the device between opposing inside surfaces. In operation, ions may be introduced, trapped, stored, isolated, and subjected to various reactions in the interior space, and may be ejected from the interior space for detection. The radial excursions of ions along the x-y plane may be controlled by applying a two-dimensional RF trapping field between opposing pairs of electrodes. The axial excursions of ions, or the motion of ions along the central axis, may be controlled by applying an axial DC trapping field between the axial ends of the electrodes. Additionally, auxiliary or supplemental RF fields may be applied between an opposing pair of electrodes to increase the amplitudes of oscillation of ions of selected mass-to-charge ratios along the axis of the electrode pair and thereby increase the kinetic energies of the ions for various purposes, including ion ejection and collision-induced dissociation (CID).
Ions present in the interior space of the electrode set are responsive to, and their motions influenced by, all electric fields active within the interior space. These fields include fields applied intentionally by electrical means as in the case of the above-noted DC and RF fields, and fields inherently (mechanically) generated due to the physical/geometric features of the electrode set. The inherently generated fields may or may not be intentional and, depending on the mode of operation, may or may not be desirable or optimal. Both applied fields and inherently generated fields are governed by the configuration (profile, geometry, features, and the like) of the inside surfaces of the electrodes exposed to the interior space. Points on the inside surfaces closest to the central axis, such as the apical line of a hyperbolic electrode, have the greatest influence on an RF trapping field and thus on the ions constrained by the RF trapping field to the volume around the central axis.
In an ideal case, the physical features and geometry of the electrodes would be perfect such that no imperfections in the active fields existed to impair the desired mode of operation of the ion processing device. The electrodes would be perfect hyperbolic surfaces extending to infinity toward the asymptotes. The response of ions to the fields would be completely predictable and controllable, and the performance of the device as a mass analyzer or the like could be completely optimized. In an ideal (pure) quadrupolar RF trapping field, no higher-order multipole fields would be present and the secular frequency of oscillation of an ion in a given coordinate direction would be independent of the secular frequency of oscillation in an orthogonal direction and independent of the amplitude of the oscillation. Moreover, the strength of the ideal field would increase linearly with distance from the central axis along either the x-axis or the y-axis.
In practice, however, the electrodes include a number of different features that engender various types of symmetrical and/or asymmetrical field faults or distortions affecting the manipulation and behavior of ions. For example, most linear electrode systems employed as ion traps eject ions from the interior space in a radial (x or y) direction orthogonal to the central axis, typically through a slot formed at the apex of at least one of the electrodes. The slot is a significant source of field faults that may be considered detrimental to the ion ejection or mass scanning process. For instance, a single slot formed in one of the electrodes generates odd-ordered multipole fields such as hexapolar fields, and two slots respectively formed in two opposing electrodes generate even-ordered fields such as octopole fields. Another source of field faults stems from the necessity that electrodes have truncated (finite) shapes that may likewise generate higher-order multipole field components. Multipoles in the trapping field may produce a variety of nonlinear resonances. In a real quadrupolar RF field employed for trapping ions, such imperfections may adversely affect the ion ejection process by causing shifts in the ion ejection time that are dependent on the chemical structure of the ions. The shift in ejection time results in mass shifts in the mass spectrum that are dependent on the chemical structure of an ion and not its mass. Therefore, it would be highly advantageous to eliminate such adverse effects when using the ion trap as a mass spectrometer.
Conventional approaches for ameliorating the undesired effects of field imperfections include increasing or “stretching” the separation of two opposing electrodes and shaping the electrodes in ways that deviate from theoretically ideal parameters. It is has been observed by the present inventor, however, that while these approaches may adequately compensate for multipole components due to the truncation of the electrodes to a finite size, they do not fully compensate for multipole components caused by large holes and slots in the electrodes. Another approach is to provide shim electrodes positioned inside of the apertures of the electrodes. See U.S. Patent App. Pub. No. US 2002/0185596 A1. This technique, however, does not address and fails to appreciate the need for, and benefits obtained from, compensating for the reduction in the field strength where the ions are oscillating, such as directly on the axis of symmetry of the slot and in the interior space of an ion processing device.
Conversely, the elimination of the effects of imperfections such as nonlinear resonances may be considered disadvantageous when performing other types of ion-processing operations such as collision-induced dissociation (CID). That is, the proper utilization of field defects may be advantageous during processes such as CID. Therefore, it would also be advantageous to be able to utilize field faults to optimize processes entailing ion excitation, such as dissociation and chemical reaction, which are carried out during periods of time in which ion ejection and mass scanning are not desired or at least should be delayed. It would also be advantageous to improve the duty cycle of CID and lessen the need for precisely balancing the voltage applied to drive the CID process with the effects of damping gas. The problems associated with the conventional requirement for such precise balancing are well-recognized in the art.